The present invention relates to geological well logging, and more particularly to methods and apparatus for testing geologic formations.
Various wireline instruments and techniques are employed in logging of uncased boreholes to measure the properties of drilled section and the fluids contained in the rock interstices for evaluating the productive capabilities of petroleum reservoir formations. After running electrical, sonic, nuclear, and other wireline logs to identify zones of interest in a borehole, a wireline formation tester may be lowered for measuring subsurface formation and hydrostatic pressures and for taking diagnostic fluid samples. At selected depths, a back-up shoe is set against one side of the borehole to press a probe or "snorkel tube" into, or tightly against, the formation on the opposite side. This provides a good seal for allowing a fluid sample from the formation to collect in a sampling chamber free of any drilling fluid.
The ability to collect a sample and the rate at which the fluid sample is recovered depend upon, among other factors, the formation's permeability, or degree to which the interstices or pores are interconnected, at the snorkel tube. In homogeneous clean sandstones and unfractured limestones, the permeability is substantially uniform in all directions, consequently, the orientation of the snorkel tube in the borehole will not affect the flow rate. In fractured formations, on the other hand, the permeability varies significantly around the borehole. Therefore, the orientation of the probe against the formation will affect the amount of recovery and the sample recovery rate.
Prior art testers make no provision for positioning the snorkel tube in the area of greatest permeability. In fact, some formation testers by their very design consistently place the snorkel tube in areas of the formation where there is least permeability. Consequently, little or no sample is obtained or the time consumed in obtaining a satisfactory quantity of a diagnostic fluid sample is unnecessarily long, and costly in terms of manhours. Even if the snorkel tube were, by chance, positioned in the area of highest permeability, its orifice communicates with a very small area of the formation, thus further limiting the sample recovery rate.